THE PHOTORECEPTOR PROCESS IN VISION 687 



10 



ts 



20 



2S 



30 



3S 



40 



FIG. 19. Dark-adaptation of the human eye measured in a peripheral area which contains both 

 rods and cones. The dark adaptation of the cones is completed within about 5 min., that of the rods 

 within about 45 min. [From VVald et al. (72).] 



S 01 



V 



Q 



30 sec bleachinq 



with strong orange liqht 



Subject J WHD 



FIG. 20. Bleaching and resynthesis of visual pigments in 

 the human fovea. Initial values after dark adaptation. Fol- 

 lowing a 30 sec. bleach with strong orange light, the density 

 is at first very low but rises rapidly in the dark. Recoscry is 

 complete in 6 min. [From Rushton (49).] 



adaptation of the cones is complete within 4 to 6 

 min., while that of the rods continues for over 45 

 min. The dark adaptation of a peripheral area of the 

 human retina containing rods and cones is shown 

 in figure 19. It is plotted in terms of log sensitivity 

 (-log threshold) the better to expose its relationship 

 to the rise of \isual pie;ment concentration. 



Another approach to this problem has been made 

 by comparing the rates of synthesis of rhodopsin 

 and iodopsin in solution. Figure 10 above shows a 

 mixture of chicken iodopsin and rhodopsin made by 

 incubating nco-b retinene in solution with a mixture 

 of cone and rod opsins. The reason the visual pig- 

 ments form separately in this instance is that iodopsin 

 is synthesized with enormously greater speed than 

 rhodopsin, about 530 times as fast at io°C (72). 

 Figure 21 shows the synthesis of the two pigments in 

 solution at 23°C. The synthesis of iodopsin is com- 

 plete within 5 min., while that of rhodopsin continues 

 for well over an hour. The data are taken from the 

 same experiment as figure 10 but with rhodopsin 

 extinctions multiplied by 1.3. It is hardly necessary 

 to labor the clo.se relationship between these measure- 

 ments, the synthesis of human rod and cone pig- 

 ments in vivo, and the course of human dark adapta- 

 tion, cone and rod. Again, however, what parallelism 

 obtains involves the comparison of log sensitivity 

 with the concentration of the visual pigments. 



One must conclude from all these measurements 

 that light and dark adaptation have their primary 

 source in the bleaching and resynthesis of the visual 

 pigments of the rods and cones. To be sure, more 

 central phenomena — changes in the sensitivities of 



